The present invention is generally directed to horizontal penetrations extending between the inner and outer walls of a cryostat, particularly one employing liquid helium as a coolant material. More particularly, the present invention is directed to a penetration plug which employs a plurality of thermally insulated nested housings which are heat stationed to several cryostat penetration structures so as to prevent large temperature gradients from occurring between the interior and exterior of the cryostat. Even more particularly, the present invention is directed to a cryostat plug for horizontal penetrations employing electrically conductive leads which extend from the penetration in normal operation (that is, leads which are non-retractable).
In the generation of medical diagnostic images in nuclear magnetic resonance (NMR) imaging, it is necessary to provide a temporally stable and spatially homogenous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures, provides an advantageous means to produce such a field. Accordingly, for such NMR imaging devices, a cryostat is employed. A cryostat contains an innermost chamber in which liquid helium, for example, is employed to cool the superconductive materials. The cryostat itself, typically comprises a toroidal structure with other nested toroidal structures inside the exterior vessel to provide the desired vacuum conditions and thermal shielding. Since it is necessary to provide electrical energy to the main magnet coil, to various correction coils and to various gradient coils employed in NMR imaging, it is necessary that there be at least one penetration through the cryostat vessel walls.
Typical prior art penetrations have been vertical. However, from a manufacturing viewpoint, the construction of vertical penetrations has produced undesirable problems of alignment and assembly. However, horizontal cryostat penetrations have not been employed for reasons of thermal efficiency. In particular, it is seen that for a coolant such as liquid helium, that there is a large dependency of density upon temperature. Accordingly, liquid helium vapor found within a vertical penetration, is naturally disposed in a layered configuration as a result of the density variation from the bottom to the top of the penetration. This layering provides a natural form of thermal insulation along the length of a vertical penetration. In particular, at any position along the axis of such penetration, the temperature profile is substantially constant. However, this would not be the case for a horizontal cyrostat penetration since any layering that would result would not be in the direction of the long axis of the cryostat penetration. Accordingly, the temperature gradient along the penetration would tend to set up free convection currents in the vapor within the penetration. This would result in a much more rapid loss of coolant than is desired. Since the cost of helium is relatively high, it is seen that this loss of coolant is undesirable.
Moreover, as a result of an as not yet fully understood phenomena, it is possible for superconductive windings within the cryostat to undergo a sudden transition from the superconducting state to the normal resistive state. In this circumstance, the electrical energy contained within the coil is rapidly dissipated as resistive (I.sup.2 R) heating of the windings. This can result in a rapid increase in internal helium vapor pressure and accordingly, any cryostat penetration must be provided with pressure relief means. Furthermore, vacuum conditions are maintained between the innermost and outermost cryostat vessels. If for some reason a loss of vacuum occurs in this volume, it is also possible to develop an increase in the coolant vapor pressure. For this reason also, pressure relief means are desirable for cryostat penetrations.
As indicated above, electrical connections must be provided through the cryostat wall to accommodate the electrical apparatus contained therein at the desired lower temperature. In some cryostat penetration designs, the electrical connections to the internal coils are made through an electrical lead assembly which is disposed entirely within an inner cryostat vessel. In such a configuration, there is a tendency for frost buildup upon the contacts and these contacts often must be heated to a temperature of about 300.degree. K. prior to making the electrical connection. It is, of course, undesirable that interior cryostat objects must be heated. It should also be understood that because of the superconducting nature of the coils disposed within the innermost cryostat vessel, that a "persistant current" mode of operation is intended. In such a mode, once the desired currents are established, the electrical power supply to the electrical elements within the innermost vessel may be disconnected. This is an advantageous mode of operation since it is highly energy efficient. However, it is seen that this method of operation exhibits the disadvantage that the electrical leads may have to be heated to provide the desired electrical contact, particularly during original magnet excitation. However, many of these problems are avoided by providing a non-retractable electrical lead assembly disposed within the penetration. However, the utilization of a non-retractable assembly introduces insulation, convection current and pressure relief problems which are not present in a retractable lead cryostat design.
Accordingly, it is seen that because of the large density changes between cold and warm helium, free convection the secondary flows are easily set up in a horizontal cryostat penetration. These flows considerably degrade the thermal efficiency of the horizontal penetration. It is also desirable to avoid the formation of frost buildup in the vapor cooled plug which could prevent the desired pressure relief. It is therefore seen that horizontal cryostat penetrations for NMR magnet cryostat require thermally efficient plugs that suppress free convection coolant vapor flow in the penetration. These plugs should also provide sufficient exhaust means to relieve internal pressure buildup in case of magnet quench or vacuum loss. Additionally, these plugs should also accommodate the utilization of non-retractable electrical leads.